High-performance porous carbon for supercapacitors prepared by one-step pyrolysis of PF/gelatin blen

J. Cent. South Univ(2012)19: 41-45DOI:10.1007/s11771-012097002 SpringHigh-performance porous carbon for supercapacitors prepared byone-step pyrolysis of PF/gelatin blendsYI Bin(易斌), ChEN Xiao-hua(陈小华), LIU Yun-quan(刘云泉), GUo Kai-min(郭凯敏)Chen Chuan- sheng(陈传盛)2, ZENG Bin(曾斌), LONG HU龙慧)1. College of Materials Science and Engineering, Hunan University, Changsha 410082, China2. College of Physics and Electronic Science,Changsha University of Science and Technology, Changsha 410114, Chinac Central South University Press and Springer-verlag berlin heidelberg 2012Abstract: a high-performance porous carbon material forapacitor electrodes was prepared by using a polymer blend methodPhenol-formaldehyde resin and gelatin were used as carbon precursor polymer and pore former polymer, respectively. The blendswere carbonized at 800 C in nitrogen. SEM. bEt measurement and Bjh mcthod reveal that the obtained carbolsses amesoporous characteristic, with the average pore size between 3.0 nm and 5.0 nm. The electrochemical properties of supercapacitorusing these carbons as electrode material were investigated by cyclic voltammetry and constant current charge-discharge. The resultsindicate that the composition of blended polymers has a strong effect on the specific capacitance. When the mass ratio of pf togelatin is kept at 1: 1, the largest surface area of 222 m/g is obtained, and the specific capacitance reaches 161 F/gKey words: mesoporous carbon; polymer blend; supercapacitor; pseudocapacitance; electrochemical propertiesmicrospores do not contribute to the total double laye1Introductioncapacitance because of a molecular sieving effect [8-9]Polymer blend technique [10-14]enables us to tailor theElectrochemical capacitors (often called super- diameter of the obtained porous carbon to formapacitors)[1-2] are promising power sources for mesopores by controlling the blending ratio of bothportable systems and automotive applications. They are polymers. The formed mesopores can provide moregenerally classified into two categories depending on the favorable and quick pathway for ion penetrationcharge-storage mechanism. The energy storage however, they have to be implemented at expense ofmechanism in electric double-layer capacitors(EDlCs) scarifying the specific surface area of the porous carbonis based on an electrostatic attraction bctween charges The low specific surface area limits their application asalong the double layer, formed at the clectrode/supercapacitor electrode materialelectrolyte interface. For the capacitors in the other classAlthough a high specific surface area is regarded ascalled pseudocapacitors, the storage mechanism in this a primary requirement for carbon electrodes to be used incase is related to fast faradaic redox reactionsEDLCs, some other aspects of the surfaceIn EDLCs, mostly based on activated carbons(ACs) physico-chemical properties can be critical to the[3-5] and considering that only the charging of the electrochemical performance Therefore, materials thatdouble-layer is involved, it should be expected that the present pseudocapacitance properties are promisinghigher the specific surface area of an activated carbon is, alternatives to EDLCs. In this case, high capacitancethe higher the capacitance is. However, observing the properties could be demonstrated by low surface arealow values of specific capacitance which were reported carbon electrodes. There is a possibility that heteroatomin Refs. [6-7] for activated carbons with surface areas such as oxygen can induce pseudocapacitive effectsranging up to 2 000-3 000 mg, one can assume that not [15-18]. RAYMUNDO-PINERo et al [19 haveall the pores are effective in the charge accumulation. reported a carbonaceous material for supercapacitorsFor example, it has been demonstrated that very narrow obtained by carbonization of a seaweed biopolymer.Foundation item: Projects(50772033, 50972043)supported by the National Natural Science Foundation of China; Project(9JJ3095)supported by theNatural Science Foundation of Hunan Province, China; Project(09A001)supported by the Scientific Research Fund of Hunan ProvincialEducation Department, China; Project(2010FJ3151)supported by the Science and Research Plan of Hunan Province, China; Projectsupported by the Science and Technology Innovative Rcscarch Team in Higher Education Institution of Hunan Province, ChinaReceived date: 2010-12-16; Accepted date: 2011-04-07CorrespondingauthorChenXiao-hua,ProfessorTel:+86-731-88822663;E-mail:enyage@sohu.comYH羋方欲42J. Cent. South Univ.(2012)19: 41-45Although the material developed has a low specific Quantachrome measuring instrument(NOVA2000)wassurface area(270 mi/g), the capacitance value per mass employed to investigate the pore properties of theof active material (200 F/g)is comparable to the best samples. The specific surface area was calculated usingactivated carbons, due to the oxygen present in the the Brunauere-Emmette-Teller(BET)equation. The porecarbon network participating in pseudofaradic size distributions were obtained from the adsorptioncharge-transfer reactions. The drawback is that the pore branch of the nitrogen isotherms by Barrette-Joyneresizes cannot be controlled effectively.Halenda(BJH)method. Finally, pore volumes wereOur strategy was to prepare a mesoporous carbon estimated to be the liquid volume of adsorption( N2)at amaterial containing a high concentration of functional relative pressure of 0.98groups by using a polymer blend method. Phenolformaldehyde resin was chosen as carbon precursor 2.3 Preparation and property measurements ofpolymer and oxygen-rich gelatin was chosen as poreporous carbon electrodeformer polymer. Pores can be controlled by changinA mixture of porous material, carbon black andPF-to-gelatin mass ratios, and a high amount of oxygen PTFE at a mass ratio of 80: 10: 10 was pressed onto ais incorporated into the carbon framework. foam nickel followed by pressing the composite in anElectrochemical properties of the obtained porousextrusion machine. At last. the electrode was dried atarbons as supercapacitor electrodes were evaluated120C for 12 h in a vacuum oven before use. Cyclicvoltagrammetry was carried out on an electrochemica2 Experimentalworkstation( CHI-660b, China) in 6 molL KOH solutionThe capacitive behavior of the composite was2.1 Materialsinvestigated by constant current charge-discharge inThe gelatin was purchased from Sinopharm Land system(Land, CT-2001A)Chemical Reagment Co, Ltd.( Shanghai, China).ANovolac-type phenol-formaldehyde polymer (PF)was 3 Results and discussionpry Changsha Zhida Chemical Ind. CoHexamethylenetetramine was purchased from ShantouFigure I shows TG curves of PF, gelatin andXilong Chemical Factory, Guangdong Province, ChinaThese chemicals were used directly without furtherpolymer blend(PF-to-gelatin mass ratio of 1: 1)innitrogen atmosphere. After heating to 900C, the carbonpurifiyields in PF and gelatin are about 58% and 18%2.2 Preparation and characterization of porousrespectively. PF shows a gradual mass loss at 250-600C and gelatin shows a rapid mass loss at 200-400oCcarbonThe porous carbon was prepared in the followingThe TG curve of the polymer blend coincides roughlyway. Gelatin was dissolved in water at 60C, and PFwith that deduced from the pf, gelatin and the blend ratio.resins were dissolved in ethanol solution including 10%Based on the tg curves. 800oC is chosen to be the(mass fraction)of hexamethylenetetramine(cross-linkingpyrolysis temperature, because at this temperature mostagent) against the mass of PF resin. Then, a PF/gelatinof the gelatin has been decomposed whereas the mass ofpolymer blend solution was formed by the addition of PFthe blends has not decreased significantly.resin into gelatin solution under continuous stirring0Subsequently, the blends were cured at 180C for 5 h ina dynamic vacuum. The blend ratios of pf to gelatin20were controlled through varying the relative mass ratioof Pf to gelatin as 4: 1, 2: 1, 1: 1, 1: 2 and 1: 4. The blendswere finally carbonized at 800C for 1 h under aPFnitrogen atmosphere The porous carbons obtained weredenoted as PG-1. PG-2. PG-3. PG-4 and PG-5PF/gelatinThe temperature of the carbonization of the blendswas determined by thermogravimetric analysis (tgaGelatinusing a dt40 Shimadzu thermal analyzer with a rate of10° C/min from room temperature to900°Cata100nitrogen flow. The morphology of the sample was2004006008001000observeed on a Hitachi $4800 scanning electronTemperature/°Cmicroscope(SEM). Elemental analysis of the sample Fig. I TG curves of Pf, gelatin and their blend (1: 1)in nitrogenwas provided by energy dispersive X-ray (EDX). atmosphereYH羋方欲J Cent. South Univ(2012)19: 41-4543As shown in Fig. 2(a), the sample before pyrolysisapparently does not have any pores on their surface. As0.012■一PG-1shown in Fig. 2(b), after pyrolysis, the surface of theo一PG-2≈0.010PG-3 becomes rough, and many pores appear in thePG-3PG-4carbon matrix. Macropores and mesopores due to the20.008PG-5evolution of decomposition gases are visible, andinduced porosity of the electrodes should be favorable0.006for electrolyte penetratione0004可0.0020102Pore diameter/nFig. 3 Pore size distributions of porous carbonTable 1 Pore texture properties and specific capacitance ofporous carboSEV+ D/ Cs0 nmSampleP(m2g )(cmg)(cmg)(cmg)nm(.g 2bPG-1930.0870.10244112PG21600.0330l110.1443.6133PG32220.05401110.1653.0161PG419800430.1390.1823.7148PG-S0.01500970.1125.0120SBET, BET surface area; Vmic, micropore volume; Vmeso, mesopore volume; Vu,total pore volume; D, average pore diameter; Csp, specific capacitance.350 mV/s25 mvs100 n210 mV/sFig. 2 SEM images of PG-3:(a)Before pyrolysis, (b) After5 mV/s2 m V/spyrolysis at800°CThe pore size distributions and the pore textureproperties of the samples are shown in Fig. 3 and Table 11It can be seen that all resultant carbon exhibits amesoporous structure. PG-3 has the highest BET surfacearea of 222 m/g, and a total pore volume of 0. 165 cm/gmeasured at a relative pressure of 0.98, most of the pores00.2040.60.81.0being mesopores with an average pore diameter of 3.0Voltage/Vnm, which are the valid pores for EDLC electrodeFig. 4 CVs of PG-3 electrode at different scan ratesmaterial [20]. In the meantime, PG-5 exhibits the lowestBET surface area of 89 m /g, with a much lower totaslightly distorted. At 25 mV/s or 50 mV/s, the shape ispore volume of 0 112 cm/g and an average porelearly distorted, which is due to the resistance of iondiameter of 5.0 nm. It is obvious that the SBer increases migration in pores because the increase of the scan ratefrom 93g of PG-1 to 222 m g of PG-3, thenaggravates the delay of the current to reach a horizontaldecreases to 89 mlg of PG-5 with increasing thevalue after reversal of the potential scan. SimilarPF-to-gelatin ratio. The composition of blendedbehaviors have been observed for PG-1. PG-2. PG-4 andpolymers shows a strong effect on the resultant surfacePG-5. Specific capacitances(Csp)evaluated from the Cvcurves are summarized in Table 1. It can be seen that theFigure 4 presents cyclic voltammograms(CVs)ofSP passes through a maximum with the change ofthe PG-3(1: 1 )electrode at different scan rates. It can be PF-to-gelatin ratios from 4: 1 to 1: 4. The highest specificfound that the current increases with the increase of the capacitance of 161 Fg is for the PG-3 with thescan rate At 2 mV/s or 5 mV/s, a rectangular shape can PF-to-gelatin ratio of 1: 1. The relatively high speciticbe observed. At 10 mV/s, the shape of the rectangle is capacitance is resulted from low specific surface area,YH羋解方欲J. Cent. South Univ(2012)19:41-45Indicating that well-developed mesoporous structure microbeads. Consequently, one may assume that theplays an important role in outstanding elcctrochemicalporous carbon obtained has a high Sext in order toperformancesdemonstrate high capacitance by low specific surfaceThe chronopotentiogram of PG-3 electrode atreddifferent charge-discharge currents is shown in Fig. 5The oxygen functional group is another importantThe regular shape of the curves indicates that the cellfactor. According to the clemental analysis, as listed inpresents a reversible charge-discharge process and that Table 2, PG-3 is constituted mainly by element carbonthe stability of the capacitor is good. Another noticeable (82.53%, molar fraction)and oxygen(17.33%, molarfeature of the charge-discharge curves is a sudden drop fraction). A small amount of CI(0. 14%, molar fraction)of potential at the beginning stage of discharge, which iscontributed to the impurity of gelatin. The initialassociated with the resistance of the cell. The good linear oxygen content of gelatin ranges between 30% and 40%relationship between the voltage and the time with (molar fraction ) The results indicate that a high amountincreasing the current density indicates that the porousof oxygen is retained in the carbon framework aftercarbon displays excellent capacitive characteristicscarbonization12Table 2 Elemental analysis of PG-310100 mA/g 200 mA/g 400 mA/gElementw/%778382.530.8217717336Cl0.14TotalI0000100000.40.2Based on the concept of polymer-blending neutralgelatin nature polymer with lots of amide groups can bereadily blended with commercially available PF resin0hrough multiple hydrogen bonding between hydroxylTimc/ksFig 5 Charge-discharge curves of PG-3 electrode at differentgroups(-OH)or hydroxymethylene groups(-CH2OH)current densitiesin the pf polymer. After pyrolysis of the pf/gelatinblends, most of the gelatin is decomposed, formingThe excellent electrochemical performance ofmesopores which provide more favorable and quickporous carbon could be ascribed to the followingpathway for ion penetration. At the same time, ancharacteristics. Firstly, according to SHI's theory [21]appreciable amount of oxygen is incorporated into thefor most carbon materials, the macropores contributioncarbon matrix. Many types of oxygen-containingto total surface area is negligible (usually less thanunctional groups such as phenolic hydroxyl2 m/g) compared to that from mesopores andcarboxyl groups and carbonyl groups could form on themicropores. In addition, the total surface area is alsosurface of porous carbon. The presence of oxygenatedconventionally divided into two parts: the microporegroups on the surface of the porous carbon can affect thesurface area and the external surface area (all of thecapacitance of the materials mainly in two differentsurface area excluding the micropore surface area).Forways: 1)The oxygenated groups can improve themost carbons, the external surface area is equivalent towettability of the carbon surface, which is very importante mesopore surface area. Instead of using one fixedto maximize the access of the electrolyte to the surface ofdouble layer capacitance for all porous surfaces, it iscarbon; 2 The presence of oxygenated groups causes thereasonable to assume that the double layer capacitancedouble-layer capacitance to arise from quick faradaicper unit micropore area(Cmic) is different from that percharge transfer reactions(pseudocapacitance)as well asfrom electrostatic charginunit external surface ( Cext). On the basis of abovediscussion, the specific capacitance(C) can be written asIt is predicted that the phenolic hydroxyl groupthe sum of two different partswhich has a weaker polarity on oxygen-containingfunctional groups than the carboxyl group, mightC=CmicSmic +Cex Sextproduce a change bias, but no catalytic effect,thuswhere Smic and Scxt are the micropore surface area andenhancing the formation of an electric double-layer(EDL)according to the following equationexternal surface area, respectively. Parameters Cmicandrext are respectively 0. 195 and 0.74 F/m for activatedC-OF+K'FC-OH/KYH羋解方欲J Cent. South Univ(2012)19: 41-45where K indicates cation and the symbol"// indicatesSOLANO A. Influence of pore structure and surface chemistry onthe adsorbed state by edlelectric double layer capacitance in non-aqueous electrolyte [J]It can be assumed that a well-adjusted balance of PFCarbon,2003,41(9):1765-177:[7] QIAO W, YOON S H, MOCHIDA I. KOH activation of needle cokeand gelatin is required to obtain an optimal performanceto develop activated carbons for high-performance EDLC []. EnergyThe content of gelatin must be high enough to favor a& Fuels,2006,20(4):16801684.large gas evolution, which develops porosity, and to [8] SALITRA G SOFFER A, ELIAD L. Carbon electrodes forobtain the largest amount of residual oxygen, whichdouble-layer capacitors 1. Relations between ion and pore dircontributes to the pseudocapacitance. The discussion[]. Journal of the Electrochemical Society, 2000, 147: 2486--2493above explains that PG-3 presents the largest specific[9] ELIAD L, SALITRA G SOFFER A. lon sieving effects in theelectrical double layer of porous carbon electrodes: Estimatingcapacitance, and although the surface area of theeffective ion size in electrolytic solutions []. J Phys Chem B, 2001obtained porous carbon is limited, the specific105(29)6880-6887capacitance is remarkable10] CHANG Kai-wen, LIM Zheng-yi, DU Fang-yi. Synthesis ofmesoporous carbon by using polymer blend as template for the high4 Conclusionssupercapacitor [J]. Diamond and Related Materials, 2009[11] PATEL N, OKABE K, OYA A. Designing carbon materials with1)Porous carbons were prepared by one-stepunique shapes using polymer blending and coating techniques [carbonization of PF/gelatin blends without any additionalCarbon,2002,403):315-320activation process, and were used as electrode materials[12] KATSUYA F, YOSHIKIYO H, TATSURO H. Estimation of porefor supercapacitorsstructures in carbon fibers prepared from polymer blends duringcarbonization by small-angle X-ray scattering []. Carbon2) The obtained porous carbons exhibit mesoporous46(4):722structures with an average pore diameter of 3-5 nm. [13 SUBRAMANIA A, KALYANA SUNDARAM N T, VUAYAAlthough the porous carbons have a relatively lowerKUMAR G. Structural and electrochemical propertics ofspecific surface area, they present largepecificmicro-porous polymer blend electrolytes based on PVdF-co-HFPcapacitance(up to 161 F/g), which is associated withPAN for Li-ion battery applications [] Journal of Power Sources2006,153(1):177-182their mesoporous structures and pseudocapacitive effect.[14] YAMAZAKIKAYAMA M. IKEDA K. Nanostructured3)The blend ratio of pf to gelatin plays ancarbonaceous material with continuous pores obtained fromimportant role in controlling the nanostructure andreaction-induced phase separation of miscible polymer blends [JIelectrochemical capacitance of porous carbons. This newCarbon,2004,42(8/9):1641-1649generationof pseudocapacitive mesoporous carbons [S] DENISAH J, MYKOLA S,GAO QL. Combined effect of nitrogen-could be very promising for advanced capacitors due toand oxygen-containing functional group of microporous activatcarbon on its electrochemical performance in supercapacitorsthe high capacitance properties, low cost and simpleAdvanced Functional Materials, 2009, 19(3): 438-447process[16] ODA H, YAMASHITA A, MINOURA S. Modification of theoxygen-containing functional group on activated carbon fiber inReferenceselectrodes of an electric double-layer capacitor [J] Journal of Power2006,158(2):1510-151[ 17] CENTENO T A, STOECKLI F. The role of textural characteristics[1 WANG Da-wei, LI Feng, LIU Min 3D aperiodic hierarchical porousng surface grougraphitic carbon material for high-rate electrochemical capacitiveperformances of activated carbons [J]. Electrochimica Acta, 2006,energy storage [J]. Angewandte Chemie, 2007, 120(2 ): 379-38252(2):560-566[2] FANG Jing, CUI Mu, LU Hai. Hybrid supercapacitor based onpolyaniline doped with lithium salt and activated carbon electrodes[18] ZHANG Chuan-xiang, LONG Dong-hui, XING Bao-lin. Thesuperior electrochemical performance of oxygen-rich activated[]. Journal of Central South University of Technology, 2009, 16(3)Is prepared from bitumiteal[J」.Ele434-439.Communications, 2008, 10(11): 1809-1811[3] KIM Y J, HORIE Y, MATSUZAWA Y Structural features necessary[19] RAYMUNDO-PINERO E, LEROUX F, BEGuin F. A highto obtain a high specific capacitance in electric double layerperformance carbon for supercapacitors obtained by carbonization ofa seaweed biopolymer J]. Advanced Materials, 2006, 18(14)[4] XU Bin, WU Feng, CHEN Ren-jie. Mesoporous activated carbon1877-188fiber as electrode material for high-performance electrochemical[20] MORIGUCHI L, NAKAWARA F, YAMADA H. Electricadouble layer capacitors with ionic liquid electrolyte [J]. Jounal ofPower Sources,2010,195(7):2l18-2124double-layer capacitive properties of colloidal crystal templatednanoporous carbons [J]. Stud Surf Sci Catal, 2005, 156: 589-594[5] LIU Ye-xiang, LI Jing, LAI Yan-qing Preparation and properties of[21] SHII I. Activated carbons and double layer capacitance [pitch carbon based supercapacitor []. Journal of Central SouthElectrochimica Acta, 1996, 41(10): 1633-1639University of Technology, 2007, 14(5 : 601-606[6] LOZANO-CASTELLO D, CAZORLA-AMOROS D, LINARES(Edited by YANG Bing)YH羋方欲